--- a/mfbt/GuardObjects.h
+++ b/mfbt/GuardObjects.h
@@ -111,17 +111,17 @@ public:
   GuardObjectNotificationReceiver() : mStatementDone(false) { }
 
   ~GuardObjectNotificationReceiver() {
     /*
      * Assert that the guard object was not used as a temporary.  (Note that
      * this assert might also fire if init is not called because the guard
      * object's implementation is not using the above macros correctly.)
      */
-    MOZ_ASSERT(mStatementDone);
+    MOZ_ASSERT(mStatementDone, "Guard object should not be used as a temporary.");
   }
 
   void init(GuardObjectNotifier& aNotifier)
   {
     aNotifier.setStatementDone(&mStatementDone);
   }
 };